One of the many problems with painting outside is time, there's not enough of it to finish our painting and get it all down before the light moves. One solution is to limit your scope of the subject, just paint what the light is doing to stuff.
But another solution is to find a way to block in the big planes of a landscape faster and one way to do that is to use a palette knife. The knife forces you to think simpler, bigger and faster. You can't mess around with washes and thin paint with a knife, it forces you to make a decision and put it down. You also can't pick around with detail when you use a knife, its not flexible enough.
Then, after you have the large areas of dark and light blocked in you can come back in with a brush to drag in broken color, soften edges and knock down some of the distractive palette knife strokes. It gives your painting more spontaneity and sense of color. The problems come when you don't separate the values enough or keep your color clean.